(a) Field of the Invention
The present invention relates to a process and a device for characterizing a sample which has at least one substance dissolved or dispersed in it, by a combination of analytical centrifuging and spectral evaluation of the inelastically scattered fraction of the scattered light emitted by the at least one substance. In particular, the invention relates to a process and a device for determining quantities characteristic of dissolved or dispersed substances in the form of particles, for example the density, molecular weight, molecular weight distribution and particle size distribution of the substances in the form of particles, providing extra information regarding the structure of the particles. In the context of the invention, "particle" is used to denote the investigated substances/materials in the form of dissolved and/or dispersed particles, this including substances both with low molecular weight and with high molecular weight.
(b) Description of Related Art
One known process for determining the characteristic quantities mentioned above is analytical centrifugation, although owing to the size of the substances which are generally to be investigated, in the form of particles, use is predominantly made of analytical ultracentrifugation. Moreover, the present application is to be understood such that other analytical centrifuges, for example a disc centrifuge, that is to say centrifuges in which only a comparatively low speed of rotation can be obtained, may also be used in the context of the process according to the invention. Nevertheless, the present invention will be explained below with reference to analytical centrifuges, and this being the case it should be noted that the term "ultracentrifuge" used in the context of the present invention always means an analytical ultracentrifuge (AUC).
In an ultracentrifuge, that is to say a very fast centrifuge with which speeds of 60,000 rpm or more can be obtained, it is possible to separate (fractionate) mixtures of substances which have different density and/or size. This being the case, ultracentrifuges can be used in a variety of ways, an overview being found, for example, in an article by W. Machtle "Analysis of Polymer Dispersions with an Eight-Cell-AUC-Multiplexer: High Resolution Particle Size Distribution and Density Gradient Techniques" in S. E. Harding et al. (Ed.) "Analytical Ultracentrifugation in Biochemistry and Polymer Science", Royal Society of Chemistry, Cambridge England 1992, Ch. 10.
The most important AUC measurement techniques are the sedimentation (S) run which fractionates the particles according to size, that is to say according to their molecular weight distribution or particle size distribution, the equilibrium run with which it is possible to determine the weight average of the molecular weight of the particles, and the density gradient (DG) run which fractionates according to density, that is to say according to chemical nonuniformity, number of components, degree of grafting, etc.
As indicated above, centrifuging can take place in a variety of ways:
1. Sedimentation run (S run)
During the S run, the sample(s) to be analyzed is or are subjected to a constant or increasing speed of rotation, starting from about 600 rpm and increasing to about 60,000 rpm. Since the sedimentation rate of the particles inside the sample is proportional to the square of the rotational speed, particles of a given size sediment 10,000 times faster when the rotational speed is increased from, for example, 600 to 60,000 rpm. Through measurement of a quantity, for example absorption, characteristic of the particles contained in the sample, it is possible to observe the kinetics of the particle sedimentation during an S run of this type. The speed at which the particles move to the bottom of the sample holder depends on their diameter. An S run therefore makes it possible to draw direct conclusions regarding the particle size and its distribution within a sample, since determination of a quantity characteristic of the sample at time T over the entire sample gives access to the "true" state of the sample in terms of the distribution of the particles within it. To this end, it is necessary to know particular auxiliary quantities, for example the particle density, the absorption coefficient or the specific refractive index increment. PA1 A particularly useful way of separating a sample which contains particles with low and high molecular weights is to carry out a layering run using layering cells, as can be obtained from Messrs. Beckman for example. In addition to at least one analysis chamber, which contains the sample, a layering cell of this type contains at least one store chamber which contains solvent. During the ultracentrifuging run, this solvent is then released as a result of the fact that, above a certain ultracentrifuge speed, the partition between the chamber containing the sample and the store chamber for the solvent is removed, and the solvent can thus enter the chamber containing the sample. PA1 During the layering run, the ultracentrifuge is thus firstly started up, then above a certain speed the solvent in the store chamber enters the sample chamber and forms a layer on the solution which it contains. During this run, the large particles sediment in the direction of gravity, while the small particles, which are incapable of sedimenting on account of their small size, remain at the interface between the sample solution and the solvent forming a layer. The effect of the presence of the extra solvent is that the band attributable to these small particles broadens since the particles partly diffuse into the extra solvent. PA1 This method is extremely useful for separating samples which contain both small and large particles together. PA1 In an equilibrium run, the weight average of the molecular weight Mw of particles can be determined. To do this, samples with different particle concentration c are centrifuged at the same time at one rotor speed until steady state conditions are reached. The relevant concentration distribution is determined from the radial concentration profile of the particles under steady state conditions within the measuring cell, and using the abovementioned auxiliary quantities which need to be determined separately, an apparent molecular weight M.sub.c is calculated. Plotting 1/M.sub.c against c and extrapolating to c=0 gives M.sub.W. Using this method, it is possible to register particles with a weight average of the molecular weight of about 300 to 1.times.10.sup.8 g/mol. Optical techniques involving an interference or schlieren method are generally used for determining the radial concentration profile in the individual samples. When schlieren methods are used, however, all other things being equal, the Z average of the molecular weight Mz is determined instead of M.sub.W. By evaluating M.sub.Z and M.sub.W it is possible to draw conclusions regarding the molecular weight distribution and therefore the nonuniformity of a sample, for example a polymer. PA1 A further option for ultracentrifuging involves mixing two solvents having different densities with the particles to be examined, and then centrifuging this mixture. The two differently dense solvents then form a density gradient in the sample, in which the particles are ordered according to their own density, which is to say they will stay at a fixed radial position where the density of the solvent mixture surrounding them corresponds to their own density. This method thus makes it possible to separate particles from one another according to their density, and thus according to their chemical nature. For example, polymer mixtures can in this way be broken down into their individual components. It is in this way possible, for example, to check that a block copolymerization has been brought successfully to its conclusion, or to determine the average and distribution of the degree of grafting in a graft dispersion.
2. Equilibrium run
3. Density gradient run (DG run)
However, although ultracentrifuging is an elegant way of fractionating samples according to molecular weight, particle size and density of the particles which they contain, this method does not in principle allow chemical identification of the fractions.
For this reason, ultracentrifuging has in the past been coupled with determination of absorption and fluorescence spectra during the ultracentrifuging run, in order to make it possible to draw conclusions regarding the "chemistry" of the particles contained in the sample. However, in particular if the particles contained in the sample are organic in nature, these methods provide only minor advantages, since the (usually) organic materials exhibit no, or at best very similar, UV/VIS absorption and/or fluorescence. Further, it is often necessary when recording fluorescence spectra to correspondingly label the particles to be examined, and this generally entails an additional working step.